This disclosure generally relates to systems and methods for detecting and assessing damage to composite structure. This process and method is compatible with but not limited to composite structures that use lightning protection systems.
Modern aircraft are being designed and built with greater percentages of composite materials. In some aircraft, more than 50% of the structural components are being manufactured with composite materials. Composite materials are tough, lightweight materials. Dominating types of composite materials, such as glass fibers, carbon fibers, aramid fibers or boron fibers, are combined with a coupling agent such as a resin, to create a product with improved or exceptional structural properties not present in the original materials. Composite materials are lighter and have better mechanical and fatigue properties as compared to aluminum. However they are also less electrically conductive and provide less electromagnetic shielding. Reduced conductivity causes reduced current dissipation which may result in damage when an electromagnetic effect, such as a lightning strike, occurs.
Specifically, when lightning hits an aircraft, a conductive path on the skin of the aircraft allows the electricity to travel along the skin and exit at some other location on the aircraft. Without an adequate conductive path, arcing and hot spots can occur, possibly affecting the skin. Also, the lower electrical shielding capability of composite materials increases the lightning threat to wiring and systems within the aircraft.
One current mechanism used to protect composite skins on aircraft against lightning strike damage is to include conductive lightning skin protection systems. Such systems may be present either in or on the composite skins of an aircraft. One type of system used to provide a conductive path on the aircraft is an interwoven wire fabric (IWWF). With this type of system, wires, such as phosphor-bronze wires are embedded in the top layer of the composite material nearest the wind-swept surface. Other types of systems may include the use of a thin copper foil. With an interwoven wire fabric system in the fuselage, the wires typically have a thickness range of about 0.003 to about 0.004 inches. These types of wires are spaced apart from each other. A typical spacing is around 0.010 inch in a 90-deg mesh pattern.
High-intensity electrical discharges, such as lightning strikes to a composite material including IWWF, may result in non-compliant properties of the IWWF within the composite material, which in turn results in a portion of the composite material that is non-compliant. Certain portions of the non-compliant composite material may not be identifiable by sight. The non-compliant IWWF must be replaced to provide electromagnetic event (EME) protection for the aircraft, including removing areas with IWWF loss and replacing the removed areas with compliant IWWF.
In addition, testing has shown that certain lightning protective structures tend to experience substrate microcracking and finish cracking. The microcracks tend to form due to repeated and extreme temperature, humidity, and pressure fluctuations. Microcracking occurs due to a number of factors including internal stresses from differences in coefficient of thermal expansion, as well as from non-optimum interface adhesion between components in composite systems.
Fiber-reinforced composite skin panels may require a localized repair to remove a portion of the panel that has been compromised. The localized repair includes removing the compromised portion of the panel, preparing the area to be repaired, generally sanding surrounding composite and edge portions in a ramped or stepped manner, fabricating, bonding and curing a composite patch that employs sufficient overlap of the composite material and interwoven wire fabric to ensure the transfer of energy from a lightning strike on the bonded repair section into the surrounding skin panel.
Different methodologies are currently being used to inspect repaired structures made of composite material. For example, U.S. Pat. No. 7,898,246 discloses a method for non-destructive inspection of a repaired composite structure comprising interwoven wire fabric. Existing processes are used to validate structural repairs, but do not validate interwoven wire fabric conductivity. In particular, they neither detect the need for maintenance or repair of interwoven wire fabric damage nor isolate or assess potential risk for EME/HIRF-related issues.
Any improvement upon the state of the art for systems and methods for inspecting and/or monitoring the health of a composite structure would be beneficial, especially if such improvement could be applied to both original and repaired composite structure.